1. Field of Invention
The present invention relates to an improved suspension system in a wheel-truck assembly for supporting a railcar that allows improved ride quality, increased resistance to suspension bottoming, and increased hunting threshold speed of a railroad car.
2. Description of Related Art
The opposed ends of a railcar body are commonly supported on spaced-apart wheel-truck assemblies for travel along a railway track. A standard railcar wheel-truck assembly generally has a laterally spaced pair of sideframes which are longitudinally operable along the tracks and parallel to the longitudinal axis of the railcar. A bolster, which is transversely positioned to the longitudinal direction of the railcar, couples the sideframes and has the car body supported on bolster center plate sections. A railcar wheel-truck, or truck, is positioned at the opposed ends of the railcar to support it during its traversal of the rail tracks.
Each sideframe includes a window portion for bolster ends and spring groups supporting the bolster, which allows bolster movement relative to the sideframe. Each spring group typically includes a plurality of coil springs extending between a sideframe spring seat portion and an undersurface of the bolster end spaced above the respective sideframe spring-seat.
Railway track conditions can include rail running surface variations or discontinuities from differential settling of track on its ballast, rail wear, corrugations, rail misalignment, worn switch frogs or misaligned switch points, as well as the intersection of rails for flange clearance, switches where switching points match with running rails, and rail joints. During normal railcar usage or operation, these and other variations can result in wheel-truck oscillations, which may induce the railcar body to bounce, sway, rock or engage in other unacceptable motions. Wheel-truck movements transferred through the suspension system may reinforce and amplify the uncontrolled motions of the railcar from track variations, which action may result in wheel-truck unloading, and a wheel or wheels of the truck may lift from the track.
The Association of American Railroads (AAR) establishes the criteria for railcar stability, wheel loading and spring group structure. These criteria are set or defined in recognition that railcar body dynamic modes of vibration, such as rocking of sufficient magnitude, may compress individual springs of the spring group at alternate ends of the bolster, even to a solid or near-solid condition. This alternate-end spring compression is followed by an expansion of the springs, which action-reaction can amplify and exaggerate the “apparent” wheel loading on the suspension system and subsequent rocking motion of the railcar, as opposed to the actual or “average” weight or load from the railcar and therein. As a consequence of the amplified rocking motion, and at large amplitudes of such rocking motion, the contact force between the rails and the wheels can be dramatically reduced on the alternate lateral sides of the railcar. In an extreme case, the wheels can elevate and misalign from the track, which enhances the opportunity for a derailment.
There are various modes of motion of a railcar body, that is bounce, pitch, yaw, and lateral oscillation, as well as the above-noted roll. In car body roll, or twist and roll as defined by the AAR, the car body appears to be alternately rotating in the direction of either lateral side and about a longitudinal axis of the railcar. Car body pitch can be considered a forward to rearward rotational motion about a transverse railcar axis of rotation, such that the railcar may appear to be lunging between its forward and reverse longitudinal directions. Car body bounce refers to a vertical and linear motion of the railcar. Yaw is considered a rotational motion about a vertical axis extending through the railcar, which gives the appearance of the car ends moving to and fro as the railcar moves down a track. Finally, lateral stability is considered an oscillating lateral translation of the car body. Alternatively, truck hunting refers to a parallelogramming or warping of the railcar truck, not the railcar body, which is a separate phenomena distinct from the railcar body motions noted above. All of these motion modes are undesirable and can lead to unacceptable railcar performance, as well as contributing to unsafe operation of the railcar.
A common apparatus utilized to control the dynamic responses of railcar trucks and bodies is a friction shoe assembly, which provides bolster-to-sideframe damping of oscillating motion. Friction shoes include a friction wedge in a pocket, which wedge is biased to maintain frictional engagement. Friction shoes dissipate suspension system energy by frictionally damping relative motion between the bolster and sideframe.
Friction shoes are most generally utilized with constant or fixed bias frictional damping structures with the friction shoe contacting complementary inner surfaces of the pockets. A retention or control spring, which biases the friction shoe and maintains it against the pocket surface and the column wear surface, is supported by a spring base or seat portion within the structure of the pocket. With a fixed or constant bias or damping spring group, the control springs do not carry load and the compression of the friction shoe assembly spring, that is the spring displacement as a function of the force, remains essentially unchanged during relative movement between the bolster and sideframe. Thus, in a constant bias arrangement, the biasing force applied to the friction shoe remains constant throughout the relative motion between the bolster and sideframes for all conditions of railcar loading. Consequently, the frictional force between the friction shoe and column wear surfaces remains relatively constant.
Alternatively, the response of friction shoes in variable bias arrangements varies with the compressed length of the retention spring. Therefore, the frictional force between the friction shoe and the column varies with the vertical movement of the bolster. However, in a variable rate spring structure, the operating range, or the spring rate, of the control spring may not be adequate to respond to the applied forces, that is the railcar weight and the oscillating dynamic forces, from variations in the track and operating conditions. In at least some variable friction force arrangements, the distance between the friction shoe and the sideframe spring seat has been considered to be adequate to accommodate a friction-shoe biasing spring with a suitable design characteristic to handle the force variations and ranges in the railcar wheel-truck assembly, even for railcars with a higher-rated, load-bearing capacity.
In fixed or constant biasing arrangements, the friction shoe frequently has a spring pocket to receive a control spring having adequate length and coil diameter to provide the requisite frictional damping.
The spring group arrangements support the railcar and damp the relative interaction between the bolster and sideframe. There have been numerous types of spring groups utilized for railcar suspension systems, such as concentric springs within the spring group; five, seven and nine spring arrangements; elongated springs for the friction shoe; and, short spring-long spring combinations for the friction shoe within the multi-spring set. These are just a few of the many noted spring arrangements that have been positioned between sideframe and bolster end assemblies. These spring assemblies must conform to standards set by the Association of American Railroads (AAR), which prescribes a fixed spring height for each coil spring at the fully-compressed or solid spring condition. The particular spring arrangement for any railcar is dependent upon the physical structure of the railcar, its rated weight-carrying capacity and the structure of the wheel-truck assembly. That is, the spring group arrangement must be responsive to variations in the track as well as in the railcar such as the empty railcar weight, the laden-to-capacity railcar weight, railcar weight distribution, railcar operating characteristics, available vertical space between the sideframe spring-platform and the bolster end, the specific friction shoe design and, other operating and physical parameters.
Prior spring group designs, such as, for example, U.S. Pat. No. 5,524,551, having a dual rate suspension system, has been limited to minimum reserve capacities of 1.50 per AAR standards S-259 and Rule 88. The only exception of spring group design with an allowed reserved capacity lower than 1.5 is railway cars specifically hauling automobiles, or autorack cars. The weight of the automobiles amounts to about ⅓of the total sprung weight of the loaded autorack cars and the suspension of the antorack cars is much softer than a suspension of the cars. Due to the added suspension of the automobiles, the natural frequency of bounce of the autorack cars splits into two frequencies: a lower frequency and a greater frequency than the natural frequency of bounce of the same car with a fixed load of the same weight. This results in reduction of the amplitudes of bounce in the operating range of speeds. A graph that illustrates how the natural frequency of bounce of an autorack car splits into two frequencies and illustrates a dynamic effect of this split on the amplitudes of the steady-state vibration is shown at FIG. 26.
More specifically, the freight car weight for a bi-level autorack, for example, is about 98,000 pounds. The vehicles shipped will weigh about 40,000 pounds to about 48,000 pounds. Thus, a fully loaded autorack may weigh in the vicinity of about 138,000 pounds to 146,000 pounds. Because of the allowable space available for the vehicles, the autorack could not reach the maximum allowable capacity of 286,000 pounds. Further, the AAR Specification M-950-AA-99 standard requires that the cars be sprung from a maximum capacity of 185,000 pounds.
The reserve capacity may be calculated by dividing the spring group total solid capacity by the total loaded weight less the “unsprung” truck weight divided by the number of spring groups. Thus, where the spring group total solid capacity for autoracks is 47,478 pounds, the total loaded weight is 185,000 pounds, the “unsprung” truck weight is 13,500 pounds and the total number of spring groups is 4, the reserve capacity is equal to 1.1. However, when calculating the reserve capacity for the actual total loaded weight of about 140,000 to 146,000 pounds, the reserve capacity will be greater than 1.4.
Further, additional suspension may be provided via a “swing motion” truck design as disclosed in U.S. Pat. No. 3,670,660. The “swinging” action between the sideframe and the bolster/transom softens the lateral accelerations. However, for higher spring loads and column forces (i.e., snubber springs) the swinging action is inhibited. So the reduced spring reserve capacity for the swing motion truck may be allowable because of the swing action.
Reducing reserve capacity for these types of loads was considered acceptable to improve ride quality of the autorack cars. With the exception of railroad cars hauling automobiles, the AAR minimum reserve capacity of 1.50 was thought to be the minimum allowable spring capacity to prevent suspension bottoming. However, the prior art did not consider the length of the car or the interaction of the suspension systems within a car. The same suspension design and damping was used for all car types.
The railcar must be physically able to bear the rated load weight and maintain contact with the track as the car travels at varying speeds along different track contours with varying track conditions. Simultaneously, the railcar and truck assemblies must have operating characteristics enabling it to be safely operable on these same varying track conditions at the unloaded, empty-car condition. Both operating weight extremes must be accommodated without posing the danger of imminent derailment for either condition.
To provide a railcar with the above-required operating range capabilities, the damping system spring group incorporated into the truck assembly must have certain static and dynamic operating characteristics. That is, operation of a car in motion on a rail track with a wide variant of track and contour conditions can lead to dynamic operating problems from oscillations, which can progress to uncontrolled instabilities of the railcar. Track-to-wheel separation is a result of several conditions, including traversal of rail imperfections, and in conjunction with the oscillation frequency of the car from traversing the non-uniform tracks, disengagement of a wheel of an unloaded railcar is not an unusual condition. Although wheel disengagement from the track does not generally result in a derailment, the implied hazard from such a separation is readily apparent and should be avoided, if possible.
One of the primary methods for dealing with the oscillations of a railcar and truck assembly is the damping from the above-noted friction shoe, as well as the stabilizing effect of the supporting springs. These oscillations may be due in part to the physical track conditions experienced by railway cars during their operation. Variations in track conditions, for example, track joints, can effect operation of the truck assembly, which track variation effects may be amplified as they are transferred through the wheel, axle and suspension to the frame. This may effect operation of the railcar as it traverses the track and encounters more of these track-induced operating problems.